Production of fine powders

ABSTRACT

An improvement in comminuting particulate inorganic oxides is achieved by vaporizing the oxides in an electric arc column in the presence of a reducing agent and thereafter contacting the vapors with an oxygen containing quench gas. The process is particularly useful in preparing submicron silica powder.

United States Patent Shaw et a1. Au 12 1975 [541 PRODUCTION OF FINE POWDERS 3,332,870 7/ 1967 Ohrbach et a1. 204/164 3,642,453 2/1972 Chilton et a1. 204/164 [75] Inventors Hen}? Shaw SCPtCh Hams; Derek 3,649,189 3/1972 Kugler et a1. 204/164 x Angler, somefvlllea both of 3,733,337 5/1973 Kugler 423/337 [73] Assignee: Exxon Research ant; Engineering FOREIGN PATENTS OR APPLICATIONS Company mden 606,732 10/1960 Canada 204/164 [22] Filed: Oct. 18, 1971 683,522 3/1964 Canada 204/164 [21] Appl' No: 190055 Primary Examiner-Edward Stern Attorney, Agent, or F irmJoseph J. Dvorak [52] US. Cl. 423/337; 423/325; 423/335;

[51] Int. Cl. B01k l/00; C01b 33/18; COlf 7/02 58 Field of Search 204/164; 423/335, 337, Improvement 111 Commmutmg Pamculate morgamc 423/325, 608 630, 635 636, 625 oxides is achieved by vaporizing the oxides in an electric arc column in the presence of a reducing agent [56] References Cited and thereafter contacting the vapors with an oxygen UNITED STATES PATENTS containing quench gas. The process is particularly use- 3 005 762 10/1961 F 204/164 fu1 in preparing submicron silica powder.

enn 3,077,108 2/1963 Gage et a1 204/164 UX 8 Claims, 3 Drawing Figures PATENTEU AUG 1 21975 FIGURE l FIGURE 3 Henry S INVENTORS Derek J. Any/er BY 3 /MORNEY PRODUCTION OF FINE POWDERS The generation and utilization of a gaseous plasma in performing chemical and physical transformations has been receiving ever-increasing commercial and scientific interest because of the ultra-high temperatures that can be reached in the plasma. Numerous devices have been developed providing plasma temperatures ranging up to 2().()0()C. and even higher. Moreover, further strides have been made in the ability of maintaining a stability of the electric are against the influence of vigorous forced convection of the feed material into the conduction column.

Among the chemical reactions that have been carried out in a plasma jet is the production of ultra-fine metal powders by reduction of the metal halides in a hydrogen plasma, and the carbothermic reduction of a refractory oxide to a metal carbide from the so-called high intensity are discharge process.

Among the physical transformations that have been carried out in a plasmajet is the comminution of a wide variety of particulate materials by vaporization and subsequent recondensation to a finely-divided powder. A specific example is the production of finely divided silica powder by entraining particulate silica into an electrical discharge with a nitrogen gas stream and thereafter contacting the vaporous feed with quench gas.

SUMMARY OF THE INVENTION The present invention provides an improved process for comminuting the size of the particulate inorganic oxide feed by vaporizing the feed in an electrical column in the presence of a reducing agent and thereafter contacting the vapors with an oxygen-containing quench gas.

The present invention is an improvement in making submicron powders such as silica powder in a plasma jet. 7

In a particularly preferred embodiment of the present invention, the particulate silica feed is forcefully projected into the conduction column of a free-burning arc discharge by entraining the silica feed in a hydrogen gas stream. Thereafter the arc effluent, containing reduced vaporous feed, is contacted with a cold stream of oxygen-containing quench gas. Preferably, the conduction column is stabilized by providing at least three anodes radially disposed around an extension of the cathode axis. Also, it is preferred to protect the cathode tip by interposing a stream of shielding gas between the cathode and the entrained solids.

By entraining the silica with hydrogen the energy rerequirements to vaporize the silica are lowered. Quenching the vaporized material with an oxygencontaining gas serves to reoxidize and condense the vaporized feed. Surprisingly, the combined features produce a material of enhanced brightness.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram illustrating the arc column contraction of free burning electric arc in the vicinity of a cathode having a conical tip.

FIG. 2 is an enlarged cross-section of an apparatus useful in interposing a shielding gas between a cathode tip and a feed material.

FIG. 3 is a schematic diagram illustrating the particularly advantageous method of stabilizing a free-burning are column by arranging tnree electrodes radially around an extension of the cathode axis.

DETAILED DESCRIPTION Referring to FIG. 1, when an arc is struck between an anode (not shown) and a cathode having a conical tip, there occurs contraction in the current carrying area in the transition region between the cathode I and the conduction column proper 2. This contraction is indicated as contraction zone 3. This contraction of the current carrying area in the transition region between the cathode 1 and the column proper 2 may also be defined by the angle alpha, a, which is determined by extending lines tangent to the column boundary at the points of inflection 25 of the contraction column. This contraction causes the natural cathode jet effect which will be explained subsequently.

The current density and therefore the self-magnetic field due to the arc current increases toward the cathode as a result of the contraction of the current carrying area. This nonuniform magnetic field exerts a body force on the electrically conducting plasma propelling it in a direction of maximum decrease in magnetic field, i.e. along the arc axis away from the cathode tip. The streaming of plasma away from the cathode tip de creases the local pressure in the immediate vicinity of the cathode tip. The pressure decrease causes the arc to aspirate gas from the surrounding atmosphere. This mechanism establishes the well known, natural cathode jet which has been observed to flow along the axis of the column away from the cathode tip in all arcs characterized by a contraction zone adjacent to the cathode.

In view of the fact that there exists an inwardly directed pressure gradient in the vicinity of the cathode tip, contraction zone 3 can serve as an injection window" through which materials may be injected directly into the arc column. Indeed, it has been found that the flow rates of the magnitude much greater than that aspirated naturally can be injected into the column through the injection window without disturbing the stability of the arc. In short, the injection of a copious stream of working fluids such as a reactive gas, and/or solids entrained in a reactive gas, into the column through the injection window is not only possible, but actually increases the heat transfer effectiveness of this part of the arc. Consequently, it is a preferred feature of the present invention in feeding particulate inorganic oxides into the conduction column of an are discharge that the particulate feed be entrained in a gas and forcefully projected in a direction substantially parallel to the conical cathode surface through the injection window. By utilizing this technique, practical amounts of material can be fed into the high temperature region of the arc discharge.

Since many materials tend to physically or chemically attack the conical tip of the cathode, it is desirable to protect or shield the cathode to maintain its integrity. Protection is afforded by interposing a stream of shielding gas between the feed material and the conical tip of the cathode.

By shielding gas is meant any gas which is not active, i.e., chemically reactive toward the cathode material, at prevailing cathode temperatures during arc operation. Typical shielding gases, especially with tungsten electrodes, are the following: helium, argon, neon,

krypton, nitrogen and hydrogen.

A convenient apparatus for interposing the shielding gas between the cathode and the feed material is shown in FIG. 2. Shielding gas is introduced via passage 15 while feed material is introduced via passage 14. The passage defining shrouds 4 and 5 form concentric orifices 34 and 35 through which fluid media can be fed into the arc column.

In order to provide a sufficient residence time for the feed powder in the conduction column, it is desirable to provide a well-defined conduction column and unidirectional tail flame. In the practice of the present invention, preferably this is obtained by arranging three anodes radially around an extension of the cathode axis and dividing the anode current equally among the three and carbon. Particularly preferred is hydrogen. Indeed, when the reducing agent is a gas, it can be used to entrain the solid feed material into the conduction colanodes so that the anode jets merge smoothly with the umn. The presence of the reducing agent permits a sigcathode jet to form a single effluent stream alon th nificantly lower temperature at which the oxide will vaextension of the cathode axis. Illustrative of this particporize and crack. For example, silica will crack to silica ular arrangement is FIG. 3, wherein three fluid transpimonoxide in an argon atmosphere at 4,000I( whereas ration anodes are arranged symetrically on a plane erif argon is used to entrain carbon and silica into the pendicular to the extension of the cath d axi sh w plasma, dissociation of the silica to silica monoxide as broken line 30. The fluid transpiration anode is a povapor g e to ompl i n at ,300K- Thus, C ndu ting rous anode through whi h ga transpires d emerges the vaporization of the particulate inorganic oxide at into the anode sheath. As shown in FIG. 3, anode 41 feeds in the presence of a reducing agent is an essential has a porous surface 41a through which gas may be fed. feature of the Present i IL In general, the gas stream transpirated through this As previously indicated, y arranging the anodes anode should be one whi h will t react ith th dially around an extension of the cathode axis, a stabiporized parti ulate feed material; h i th i lized conduction column with a unidirectional tail stant process the reducing gas in whose presence th flame is obtained thereby assuring that the residence vaporization reaction is conducted may be employed. time Of the Powder n h plas iS ufficient for Com- Gases that are suitable for transpiration through the an- Plete vaporization of the feed- Contacting the p rd in lud h lium, argon, nitrogen and hydrogen. ized feed at the end of the tail flame with an oxygen- While the present invention is being illustrated with containing queheh g results in l'eoxidatioh 0f the fl id t i ti anodes non-consumable anodes or duced feed, e.g. silicarmonoxide is reoxidized to silica consumable carbon anodes may be em loyed. I d d, and the vaporized material is condensed in veryfinely if the vaporization is to be conducted in the presence divided formy Oxygen-containing as may be used of carbon as a reducing a e t, a carbon consumable but it is particularly advantageous in the practice of the anode could be employed to great advantage. I-Iow- Present ihVefltiOn t0 us ailever, use of a consumable carbon anode as the source The Powder in the quench Product gas may be p of reducing agent in the conduction colum i t rated by conventional means such as cyclone filter bags ti la l refe d, 40 and electrostatic precipitators and the like. The recov- The number of anodes also can be varied. For examered Powder-free gas can be l'eebmbtessed and ple, a single anode in the shape of an annular band can eled for use in the Plasma and/ in the quench gas b l d streams, if not burned or otherwise discarded.

Any arti lat inorganic id f d may b In the practice of the invention silica was commijected into the high temperature region of the conducnuted in a conduction column generated by a conical tion column and there vaporized. Typical of such matecathode and three fluid transpiration anodes arranged rials are silica, magnesia, zirconia, alumina and the like. in the geometry as illustrated in Hydtbgeh was Since solids are entrained in a gas stream, it is preferaused as the working 8 as as the Powder ble that solid feed material have particles below 80 miand Shielding 8 was used as a quench g and a crons in size, preferably the particles ranging in size White q r z Sand P r l/+270 mesh Size) was f b t 50 t 70 microns Th particles h i used as feed to produce submicron silica powder. Forthese ranges of sizes are generally available in the comparison data from another run using argon solely trade, for example, sand having particle sizes ranging as the Working gas and Powder carrier gas is given in from 50 to i i ll fl bl and can be 5 the table below which also lists the parameters and reused in the production of submicron size silica. sults of these tests- TABLE I Parameters Run 1 Run 2 Arc current, amps 360 360 Arc voltage. volts I45 of Power used in Arc 87 87 Are gap, startup 1/4 1/4 Arc gap, operation 6" 6" as flows Anode gas, argon standard cubic ft/hr. 1 10 U0 Shielding and powder gas carrier Ar I-I2 Shielding and powder gas carrier flow standard cubic feet/hour 3O 52 TABLE I CONTINUED Parameters Run 1 Rt: 1 2

Quench gas. air standard cubic ft/min. 20 2O Feed powder rate, --l70/+270 mesh silica.

pounds per hour 7 7 Consumed kilowatt hours per pound of product 14 7: Feed vaporized 32 c5 Particle size range. ,u. .005-0.05 DOS-0.05

As can be seen from the foregoing data, the comminution of the inorganic oxide solids in the presence of the reducing agent makes it possible to operate the process at lower specific power. Additionally, it has been discovered that the process of the present invention rent-carrying area in the transition region in the vicinity of the cathode;

forcefully projecting a particulate inorganic oxide along said conical tip of said cathode into and through said contraction of the current-carrying 15 provides a cleaner product. This IS demonstrated in the area i id are di h hil simultaneously table below where Munsells Value, a measure Of the forcefully projecting a reducing agent along aid brightness of c np compared with e brlghtness conical tip of said cathode into and through said of the Standard glven for Pmducts havmg dlfiel'em contraction of the current-carrying area into said origins. The values were obtained using a Model DJ are discharge; and C0101 y? sold y Konmorgen Corporation Give" in thereafter contacting the vapors with an oxygen conthe table is the Munsell Value of a commercially availmining quench able silica produced by burning silica tetrachloride in The process f l i 1 h in the reducing is sethe presence of hydrogen and Oxygen lected from the group consisting of hydrogen, aliphatic TABLE II hydrocarbons and carbon.

3. A process of claim 1 wherein the oxygenscuicc of Silica Munsell's Value containing gas is air.

4. The process of claim 1 wherein the reducing agent Plasma et with hydrogen 90.1 h d Plasma jet with argon 85.] y rogen- Commercially available material 5. The process of claim 1 wherein said particulate inorganic oxide feed is silica. h l 6. The process of claim 1 wherein said inorganic w at 15 C alme oxide is selected from oxides of silica, magnesia. zirco- 1. In the process of commmutmg a particulate inornia and alumina game. Wilde by vaponzmg the morgam? oxlde wlth a 7. The process of claim 1 wherein said particulate in- P Jet i slibsequerifly recondensmg organic oxide has a particle size below 80 microns. lzed p i oxlde f compnsmg: 8. The process of claim 1 wherein said inorganic provldmg adfree g g g dlschafgel g i a oxide is entrained in said reducing agent and forcefully anode a cat 0 e avmg a w ere y projected along said conical tip of said cathode. said are discharge forms a contraction zone of cur- 40 

1. IN THE PROCESS OF COMMINUTING A PARTICULATE INORGANIC OXIDE BY VAPORIZING THE INORGANIC OXIDE WITH A PLASMA JET AND SUBSEQUENTLY RECONDENSING THE VAPORIZED INORGANIC OXIDE, THE IMPROVEMENT COMPRISING: PROVIDING A FREE BURNING ARC DISCHARGE BETWEEN AN ANODE AND A CATHODE HAVING A CONICAL TIP WHEREBY SAID ARC DISCHARGE FORMS A CONTRACTION ZONE OF CURRENT-CARRYING AREA IN THE TRANSITION REGION IN THE VICINITY OD THE CATHODE, FORCEFULLY PROJECTING A PARTICULATE INORGANIC OXIDE ALONG SAID CONICAL TIP OF SAID CATHODE INTO AND THROUGH SAID CONTRACTION OF THE CURRENT-CARRYING AREA INTO SAID ARC DISCHARGE WHILE SIMULTANEOUSLY FORCEFULLY PROJECTING A REDUCING AGENT ALONG SAID CONICAL TIP OF SAID CATHODE INTO AND THROUGH SAID CONTRACTION OF THE CURRENT-CARRYING AREA INTO SAID ARC DISCHARGE, AND, THEREAFTER CONTACTING THE VAPORS WITH AN OXYGEN CONTAINING QUENCH GAS.
 2. The process of claim 1 wherein the reducing is selected from the group consisting of hydrogen, aliphatic hydrocarbons and carbon.
 3. A process of claim 1 wherein the oxygen-containing gas is air.
 4. The process of claim 1 wherein the reducing agent is hydrogen.
 5. The process of claim 1 wherein said particulate inorganic oxide feed is silica.
 6. The process of claim 1 wherein said inorganic oxide is selected from oxides of silica, magnesia, zirconia and alumina.
 7. The process of claim 1 wherein said particulate inorganic oxide has a particle size below 80 microns.
 8. The process of claim 1 wherein said inorganic oxide is entrained in said reducing agent and forcefully projected along said conical tip of said cathode. 